Positive displacement pumps can be used to add fluid pressure under certain operating conditions. A supercharger is one type of positive displacement pump that is used to boost air pressure at an engine air intake. Positive displacement air pumps typically have meshing, multi-lobed rotors within a rotor housing. Air is moved from an inlet to an outlet; clearance between the rotors and the rotor housing is designed to prevent air from following unintended paths. Air leakage around the rotor end faces is one unintended path and a cause of positive displacement air pump inefficiency.
The rotors are mounted on rotor shafts. The rotors and rotor shafts may tend to expand and contract due to thermal fluctuations. The rotor housing may also tend to expand and contract, and may do so at different rates than the rotors or rotor shafts, especially if formed from a different material. One solution has been to leave a gap between the rotor face and the rotor housing at the inlet end of the housing that is sufficiently large to allow the rotor shafts and the housing to expand relative to one another. The rotor shafts are typically fixed axially to the rotor housing at one end by bearings, referred to herein as axial bearings. Needle bearings between the rotor shafts and the rotor housing on the other end allow the rotor shafts to expand and contract axially relative to the rotor housing.